As a light emitting device conventionally used as an illumination device, a discharge-type fluorescent lamp and an incandescent lamp and so forth are given as examples. However, in recent years, a light emitting diode that emits white light has been developed. The white LED lighting unit serves as an ideal illumination device having advantages that the generation of heat is reduced, luminous efficacy is excellent, power consumption is reduced, having long service life without blowing filament like the conventional incandescent light bulb, and capable of eliminating a harmful material such as mercury having an influence on environment, which is indispensable to the conventional fluorescent lamp.
As required elements as a light source for a general lighting unit such as the aforementioned white LED lighting unit, the elements of luminance and color rendering properties are given as examples.
First, explanation will be given to the element of luminance representing the brightness of the light source.
For example, even if the intensity of the emission emitted from the light source is physically same, compared with a yellow light and a green light, a blue light of a short wavelength side and a red light of a long wavelength side are felt darker for human eyes. The reason is that a yellow-green light at about 555 nm wavelength most brightly responds to the visibility of human beings. Therefore, when the light emitted from the light source has larger emission component at 555 nm or around, even the same color tone becomes visually brighter, and an index to indicate the brightness of the light source for human vision is the luminance.
Next, based on the color of the object responding to human vision when solar light is applied on an object, the color rendering properties are expressed as a value of reproducibility (approximation) of color of the object responding to human vision when lights from the light source is applied on the object.
According to JIS Z 8726, the color rendering properties of the light source is expressed in a numeric value by a general color rendering index (Ra). This is the value by which the difference between the color of a color rendering reference sample when illuminated by a sample light source, and the color of the reference sample when irradiated with a reference light analogous to a natural light is evaluated. When there is no difference between above colors, showing absolutely the same color, the color rendering index is expressed by 100. Even if having the same color temperature of the light source, the way of looks of color due is varied according to the color rendering index, and therefore if the color rendering index is low, the color due looks darker and dull. Therefore, the light source which emits light over the whole visible light region can be the light source with good color rendering properties.
At present, in the white LED lighting unit becoming widely used as a general lighting unit, generally the LED emitting blue light and a phosphor emitting yellow light excited by receiving the blue light are combined to obtain an emission that looks white for human eyes by a synthesized emission wavelength of the blue light and the yellow emission. However, in this system, since the emission that looks white is formed of the blue light and the yellow emission, green and red parts required as the light source for lighting unit become insufficient. A problem involved therein is that the color rendering properties are deteriorated. Particularly, a color component of a long wavelength side of the visible light region, that is, the component of red color is insufficient. Therefore, the emission looks slightly bluish white, although it looks white at first glance. Accordingly, when the white LED lighting unit is used as the general lighting unit, as for a red object, it looks significantly dull red and deteriorated in color rendering properties, thereby being improper as the light source. In addition, lack of the red emission component makes it impossible to obtain slightly reddish white emission such as warm white of 4500K or less in a correlated color temperature.
In order to improve the color rendering properties of the aforementioned white LED lighting unit, the white LED lighting unit has been developed, having a structure that the blue LED, the phosphor excited by receiving the blue light emitted by the LED to emit yellow light, and the phosphor excited by receiving the blue light emitted by the LED to emit red light are combined, to obtain the white light by the combination of the blue light, the yellow emission, and the red emission. Also, the white LED lighting unit improved in color rendering properties has been developed, having another structure that the blue LED, the phosphor excited by receiving the blue light emitted by the LED to emit green light, and the phosphor excited by receiving the blue light emitted by the LED to emit red light are combined, to obtain the white light by the combination of the blue light, the green emission and the red emission. It is considered that the white LED lighting unit thus constructed to obtain the white light by combination of the blue LED and a plurality of phosphors, is capable of obtaining an arbitrary emission color other than the white color, according to the combination and mixing ratio of the phosphors.
However, the aforementioned white LED lighting unit by combination of the conventional blue LED and phosphors has a problem that the emission efficiency of the red phosphor used therein is low and the emission is therefore dark. Therefore, when manufacturing the white LED lighting unit, the mixing ratio of the red phosphor must be increased out of the mixing ratio of the phosphors, compared with the phosphors of other colors. This causes the mixing ratio of the phosphors of other colors to be reduced, thereby making it impossible to obtain the white light having a high luminance this time. Particularly, in the white LED lighting unit, in which emission component of the long wavelength side (red color side) occupies a large ratio and the light of low color temperature such as warm white is required, necessary quantity of the red color phosphor is further increased. Therefore, compounded quantities of the phosphors of other colors are relatively further decreased, and the yellow-green light having an emission spectrum with a peak wavelength at about 555 nm which looks brightest for the visibility of human beings becomes relatively insufficient. The problem conspicuously involved therein is that the luminance in the entirety of the white LED lighting unit is further deteriorated.
Therefore, in recent years, in order to solve the problem that, in the case of being excited by the light having emission spectrum with a peak wavelength at long wavelength range, the emission efficiency of the aforementioned red phosphor is lower than that of the phosphors of other colors, as the red color phosphor having an excitation spectrum with good excitation band on the long wavelength side and having an emission spectrum with wide half-value width, the phosphor containing nitrogen, for example, silicon nitride such as (Ca, Sr)2Si5N8:Eu is reported. Then, by combining the blue light emitted from a blue LED, the yellow light emitted from a phosphor represented by YAG:Ce, which emits yellow light when excited by the blue light emitted from the LED, and the red light emitted from the red phosphor containing the nitrogen, which is excited by receiving the blue light emitted from the LED to emit the red light, the illumination device with increased luminance and further improved color rendering properties is proposed (see patent documents 1 to 4).
(Patent Document 1)
PCT Japanese Publication No. 2003-515655
(Patent Document 2)
Japanese Patent Publication No. 2003-277746
(Patent Document 3)
PCT Japanese Publication No. 2004-505470
(Patent Document 4)
WO2004/039915 pamphlet A1 number
By combining the aforementioned LED emitting blue light, yellow phosphor such as YAG, and nitride red phosphor, or by combining the aforementioned LED emitting blue light, green to orange phosphor, and nitride red phosphor, the white light whose luminance is secured and having good color rendering properties can be obtained in the white light exceeding 5000K in correlated color temperature.
However, according to the study of inventors of the present invention, the problem is that in the region of 4500K or less in correlated color temperature, it is difficult to obtain the light having a sufficient luminance even by a method of combining the aforementioned blue LED, yellow phosphor, and nitride red phosphor. Particularly, the problem is that in the slightly reddish warm white of about 3000K in color temperature, sufficient luminance can not be obtained, thus being felt dark for human beings.
Also, it is found that even when the white light in the region of 4500K or less at correlated color temperature is obtained by the method of combining the aforementioned blue LED, green to orange phosphor, and nitride red phosphor, the light having the good color rendering properties can not be obtained. It is considered as a reason therefore that the light on the side of long wavelength of 630 nm or more is insufficient in the white light.
Therefore, the inventors of the present invention study on the known red phosphor also other than the aforementioned nitride red phosphor. However, it is found that any of the red phosphors can not exert sufficient luminance and obtain the light having a high color rendering properties, with its low emission efficiency, when the blue LED is emitted as the excitation light.
Meanwhile, it is considered that the light source such as LED lighting unit is desired, producing light of white or other desired colors having the sufficient luminance and excellent in color rendering properties, even in the light of not only the white color having high color temperature but also the warm white having low color temperature. However, it is found that in the light emitting device in which the phosphors such as the nitride red phosphor according to the conventional technique are combined using the blue LED as the excitation light, it is insufficient to have a sufficient luminance and further obtain the emission improved in color rendering properties.